1. Field of the Invention
This invention pertains to eyeglasses and more particularly to eyeglasses incorporating adjustable focus lenses. Most particularly, this invention pertains to eyeglasses of the type described wherein focus is effected biomechanically.
2. Statement of the Prior Art
Adjustable curvature lenses for accommodating adjustable focus are known in the art. As disclosed in the November, 1940 issue of the Journal of the Optical Society of America (JOSA), adjustable focus lenses generally take one of two forms. In one, two thin spherical segments are held together at their peripheries and the cavity therebetween filled with a clear liquid. By compressing the spherical segments at their peripheries, say by means of a retaining ring, the curvature, and hence the focal length, of the lenses may be varied. The other disclosed embodiment is similar in construction to the first except that the focal length of the lens is varied by controlling the fluid pressure in the cavity. Fluid lenses of the type described in the JOSA article and applications therefor may be found in U.S. Pat. Nos. 2,300,251, 2,836,101, 3,588,235 and 3,641,354. Such lenses have been proposed for opthalmic correction, particularly as an aid for persons suffering from presbyopia, the age-related inability to focus on objects in the near visual field.
Following present practice, persons suffering from presbyopia are fitted with reading glasses. While satisfactory for their intended purposes, such glasses must be removed to perform visual tasks beyond the reading range. Dual focus lenses have been substituted to avoid this inconvenience, the user selecting the near or far focus portion of the lenses by appropriate head tilt. Where correction for additional viewing distances is required, trifocal lenses have been used. While such multi-zone lenses are convenient for their intended tasks, such arrangements are inconvenient or worse when the position of the object necessitates uncomfortable or impossible head tilt angles. Recognizing this deficiency, Allen, U.S. Pat. No. 3,365,263, proposed a manually controlled inflatable nose pad disposed beneath the bridge of conventional bifocals for raising and lowering the frame and lenses. However, this device is quite cumbersome and, in any event, does not overcome the focusing limits inherent in fixed focused lenses.
One suggested solution to these problems is disclosed in Wright's U.S. Pat. No. 3,598,479 which reveals a pair of eyeglasses incorporating adjustable focus fluid lenses. More particularly, each lens of Wright's eyeglasses defines a flexible liquid-filled cell which communicates with a common finger operated pump control located on the frame. Thus, by manually operating the pump, the liquid pressure in the cells is elevated, and the resulting flexing of the cells increases the curvature of the lenses.
Unfortunately, Wright's principal feature, an adjustable focus lens centered in the optical field of each eye, is rendered a burden by the "hands-on" focusing arrangement which requires attention not only when visual tasks change, but also to accommodate changes in focus resulting from changes in temperature, etc. This burden is, of course, compounded when hands are otherwise occupied, dirty, wet, gloved, etc. Another disadvantage of Wright's eyeglasses results from equalizing cell pressures in an attempt to effect corresponding focus of both lenses. Such equalization ignores differences in the pressure response of the resilient members resulting from age, manufacturing differences, environmental effects, etc, and results in unacceptable differences in the focal lengths of the respective lenses.